1. Field of the Invention
The field of the present invention relates to civil aviation and, in particular, to noise generation by planes.
2. Description of the Related Art
The noise generated by civil planes, notably at take-off, is a widely known disturbance and numerous innovations have been imagined to try to reduce it. One of the main sources of this noise lies in the jet of the engines, which are used at full power during the phase of take-off. Of course, important works have been carried out to try to reduce the noise of the reactor jet, as for example saw-toothed chevrons for the exhaust nozzles: the hot gases one, from the main stream of the reactor, and the nozzle for said cold gases which come from the by-pass air of the engine.
To limit the disturbances undergone by the side residents of airports, strict standards have been imposed, which limit the noise which can be heard in different points situated around the plane, at various distances and in several directions with regard to the take-off runway.
One of the particularly critical points to be taken into account by plane designers, in terms of acceptable maximal noise to obtain the certification of a plane, is in a lateral position with regard to the plane, at a distance of 450 m from the take-off runway. The presence of the pylon, i.e. the mast which supports the engine and fastens it to the wing, locally generates, at the place of the gas exhaust, high levels of turbulence in the flow, with consequently a very significant increase of the side noise from the engine. This phenomenon is particularly acute for the configurations where the pylon is prominent beyond the plane of exhaust of gases, which becomes a very frequent configuration in the recent civil planes.
The results of numerical calculations or measures made with models in a wind tunnel clearly show that the effects of interactions between the stream circulating around the pylon and the pylon itself generate a considerable increase of the levels of turbulence and, as a consequence, of the noise level. An important modification of the angular development of the jet radially around the pylon can be also noted, which tends to direct the reactor jet around the pylon, in the direction of the wing.
Besides, the acquired experience shows that the introduction of the pylon, in addition to its influence on the sound level increase in a configuration of conventional exhaust, can also considerably reduce the efficiency of others devices provided for reducing the noise of exhaust gases, such as chevrons or mixers provided on the nozzles.
So, the presence of the pylon introduces, in terms of acoustics, an increase of the exhaust noise at the side point of certification, which can vary between 2 and 3.5 EPNdB (Effective Perceived Noise, or level of the effectively heard noise, in decibels) according to the engine cycle, the size of the pylon and the considered geometries of exhaust. In reference to FIG. 1 showing a noise curve BM of a turboshaft engine without pylon and a noise curve BMP of a turboshaft engine with pylon, the presence of the pylon generates turbulent kinetic energy in excess downstream therefrom. The turbulences form small whirling structures which grow and irradiate outside the turboshaft engine at low frequency and generate noise.
The need to reduce the jet noise being a constant concern of engine manufacturers, the interest to reduce the noise at source is obvious, that is by acting on the local turbulent flows around the pylon. In fact, the potential of noise reduction even seems more important than the one induced by the implementation of chevrons or micro jets to the nozzle periphery.